Variable inductor and method

ABSTRACT

A coil form for electrical inductances is provided which is impregnated with a flame-retardant resin and cured but which maintains sufficient flexibility and resiliency to permit operations such as forming, punching, notching, slitting and the like after curing. The coil form is formed by a plurality of layers of spirally wound strips of kraft paper which enclose one or more layers of spirally wound strips of high temperature resistant polyamide. After the tube is formed it is impregnated by immersing in a vat of flame-retardant phenolic resin and cured. Thereafter a coil winding is placed on the tube. A core is placed within the tube to form a variable inductor.

United States Patent Hatton [151 3,707,693 51 Dec.26, 1972 [54] VARIABLE INDUCTOR AND METHOD [72] Inventor: Richard L. Hutton, North Barrington, Ill.

[73] Assignee: Precision Paper Tube Company,

Wheeling, ll].

[22] Filed: April 19,1971

21 Appl. No.: 135,015

, I52 U.S. Cl. ..336/136, 29/605, 138/144,

[51] Int. Cl ..H0lf 21/06 [58] Field of Search ..336/136, 208; 156/190, 194; 161/229; 138/144; 29/605; 174/110 N [56] References Cited UNITED STATES PATENTS 3,267,968 8/1966 F011 ..336/208 3,314,133 4/1967 Stahl 3,367,816 2/1968 Mills 156/190 OTHER PUBLICATIONS Nomex Properties & Performance DuPont Brochure, Sept. 1965, copy 174-110 N Primary Examiner-E. A. Goldberg AttorneyDawson, Tilton, Fallon & Lungmus [5 7] ABSTRACT A coil form for electrical inductances is provided which is impregnated with a flame-retardant resin and cured but which maintains sufficient flexibility and resiliency to permit operations such as forming,,

17 Claims, 6 Drawing Figures PATENTEDnc2s m2 INVENTOR. RICHARD L. HATTON BY flw aw aam m ATT'YS BACKGROUND AND SUMMARY This invention relates to coil forms for use in forming electrical inductances, and more particularly, to flameretardant impregnated coil forms.

Flame-retardant impregnated tubes or coil forms have heretofore generally been formed from natural dielectric kraft paper which is spirally wound on a round mandrel or arbor to form a generally cylindrical tube. The tube is then impregnated with flame-retardant phenolic resin by immersing the tube in a vat of resin, and the impregnated tube is cured in an oven. The resultant coil form, while being flame-retardant, is extremely brittle, and subsequent operations such as forming, punching, notching, slitting and the like are extremely difficult to perform without cracking or breaking the tube. Further, impregnated flame-retardant tubes have generally been available only in circular cross-sections. If the paper strips were wound on a rectangular mandrel prior to impregnation and curing, the spirally wound strips would have a tendency to twist along the axis of the tube during curing to assume a more circular configuration. Forming a rectangular tube from a round tube after curing generally has not been practical since the cured impregnated tube is usually too brittle to survive such forming.

Coil forms have been made from synthetic materials such as nylon and other polyamides, but tubes made of these materials are considerably more expensive than tubes made of kraft paper. Further, although these tubes have sufficient flexibility to be formed, the resilience of some of these materials makes it difficult to maintain a formed configuration. For example, if a round tube were formed into a rectangular configuration, or were provided with embossments, the tube might tend to reassume its original round shape or lose its embossments.

The invention provides a flame-retardant resin-impregnated tube formed of spirally wound kraft paper with one or more layers of a high temperature resistant polyamide such as nylon interposed between layers of paper. The resultant tube can be impregnated with a flame-retardant phenolic resin and cured and will retain sufficient flexibility to satisfactorily undergo various operations such as forming, punching, notching, slitting, etc. The tube is also more dimensionally stable than other resin-impregnated paper tubes, and the final dimensions of the tube canbe controlled relatively accurately. A flame-retardant, noncorrosive, resin-impregnated rectangular tube can be formed by first forming a circular tube, impregnating with resin, and curing. Thereafter, the cured circular tube can be hot-formed into a rectangular shape without destroying the tube, and the formed tube will maintain its rectangular configuration.

DESCRIPTION OF THE DRAWING The invention will be explained in conjunction with illustrative embodiments shown in the accompanying drawing, in which;

FIG. 1 is a perspective view of a tube being formed in accordance with the invention;

FIG. 2 is a longitudinal sectional view of a completed tube;

FIG. 3 is a perspective view showing the steps of forming a circular tube into a rectangular tube;

FIG. 4 is a fragmentary sectional view showing a circular coil form provided with a tuning core.

FIG. 5 is a sectional view taken along the line 5-5 of FIG. 4; and

FIG. 6 is a fragmentary sectional view of another embodiment of the invention.

DESCRIPTION OF SPECIFIC EMBODIMENTS Referring to FIGS. 1 and 2, the numeral 10 designates generally an elongated tubular coil form to the joints of the adjacent layers. Polyvinyl acetate or equivalent adhesive may be used to adhesively join adjacent layers.

The strips 11 and 13 which form the inner and outer layers are formed of natural dielectric kraft paper, and the strip 12 is formed of a high temperature resistant polyamide such as nylon. I have found that Nomex nylon paper, type 4-1 1, available from E. I. du Pont de Nemours & Co. gives particularly good results. Nomex nylon differs from conventional nylon in that it has better strength and stability at temperatures in excess of the melting temperature of nylon (about 250 C.). Nomex nylon still has half its strength at 285 C., and it is difficult to ignite even with direct flame.

After the tube is formed, it is impregnated by immersing in a vat of conventional flame-retardant phenolic resin. After the tube is impregnated, it can be cured in an oven in the conventional manner. Good results have been obtained by curing at about 250 to about 275 F. for about 4 hours.

I have found that even after the tube is impregnated and cured, the intermediate layer of Nomex paper provides the tube with sufficient flexibility to permit the tube to be punched, notched, slit, and to undergo other operations which may be required to adapt the coil form for use in providing an inductance and to integrate the inductance with other components of a circuit. The fibrous structure of the Nomex paper strengthens the tube and permits lugs or other wire anchoring means to be inserted securely into the tube without fracturing the tube. Once the lugs are inserted, the fibrous intermediate layer serves to retain the lugs in the tube.

After the desired mechanical operations are performed on the tube, an inductance can be formed by winding an electrical conductor on the tube. The use of the intermediate layer of Nomex paper increases the dimensional stability of the tube over the dimensional stability of impregnated kraft tubes, which have a tendency to shrink during curing, and the final dimensions of the coil form can therefore be relatively carefully controlled. It will be appreciated that the dimensions of the coil wound on the coil form can affect the electrical characteristics of the inductance, and it is desirable to be able to predict with a substantial degree of accuracy what the dimensions of the coil form will be after cur- I have also been able to provide for the first time a flame-retardant, non-corrosive impregnated rectangular coil form. A circular coil form is first formed as shown in FIGS. 1 and 2 by winding an intermediate layer of Nomex paper between layers of kraft paper, and the resultant tube is impregnated and cured as hereinbefore described. Thereafter, the circular tube is hot-formed into a tube having a rectangular cross-section as shown in FIG. 3. A round tube is advanced axially toward a die 18 by a pilot or ram 19. The pilot has a bullet-shaped nose 20, a rectangular holding section 21 which can be received by the round tube, and an enlarged section 22 which pushes the tube into the die. The die 18 is provided with a central bore which has a frusto-pyramidal forming portion 23 and a second or forward portion 24 which has a relatively constant rectangular cross section. The perimeter of the second portion 24 of the bore is substantially the same as the circumference of the circular tube, and the perimeter of the frusto-pyramidal portion at the rearward end thereof is such that the round tube can be inserted into the die without difficulty. As the tube is pushed forwardly by the pilot, the diminishing cross section of the forming portion gradually forms the tube from a circular to a rectangular configuration. When the holding portion of the pilot is fully inserted into the die, it is retracted to pick up another tube, and each tube within the die is pushed forwardly by the tube immediately therebehind. A fully formed rectangular tube is shown leaving the die at 10'.

The die is heated, as by heating elements (not shown) contained within the walls of the die, as the tubes are pushed through, and in one specific embodiment the die 18 was inches long and was heated to a temperature of about 250 F. to about 350 F. One inch long circular tubes were pushed sequentially through the die at the rate of about 1000 per hour, each tube pushing the tube immediately ahead of it.

The hot-formed rectangular tube is also dimensionally stable and will retain its rectangular configuration without any substantial tendency to return to its original circular configuration.

Referring to FIGS. 4 and 5, the exterior of a circular coil form 25 formed in accordance with the intervention can also be indented or punched as at 26 to provide embossments or projections 27 on the inner wall of the tube which can cooperate with the threads of an externally threaded tuning core 28 of powdered iron or the like to permit the core to be threaded into and out of the coil form to provide a tunable inductance. Heretofore, flame-retardant resin-impregnated pure kraft tubes were generally. too brittle to be so embossed, and embossments in tubes made of synthetic materials often do not provide sufficient torque because of the flexibility and resilience of the material.

After the rectangular coil form is formed, a suitable inductance can be provided by winding an electrical conductor thereon.

A modified spirally wound coil form 29 is shown in FIG. 6, the coil form being formed by three inner layers 30, 31 and 32 of spirally wound strips of kraft paper, two adjacent layers 33 and 34 of spirally wound strips of Nomex paper, and three outer layers 35, 36 and 37 of spirally wound kraft paper.

I have found that a single layer of Nomex paper generally provides sufficient flexibility to permit conventional mechanical operations to be performed on the tube for tubes having about 3 to about 5 layers of kraft paper. If a greater wall thickness is desired, two or more layers of Nomex can be used, and it is preferable to wind the Nomex layers in about the middle of the tube. The outermost and innermost layers should be kraft paper rather than Nomex paper, and the number of Nomex layers should be less than one-half of the total number of layers of the tube.

Several specific embodiments of tubes have been made with Nomex strips having thicknesses of 4, 6, and 9 mils and kraft strips having thicknesses of 3, 5, 7, l0, and 15 mils.

Tubes formed in accordance with this invention can be priced competitively with conventional flame-retardant impregnated tubes formed entirely of kraft paper, and they enjoy much greater flexibility, resilience and dimensional stability which permits the desired mechanical operations to be satisfactorily performed.

While in the foregoing specifications I have described specific embodiments of my invention in considerable detail for the purpose of explanation, it is to be understood that many of the details herein given may be varied considerably by those skilled in the art without departing from the spirit and scope of my invention.

Iclaim:

I. An electrical inductance comprising a hollow tubular member and an electrical conductor wound on the tubular member, the tubular member comprising a plurality of adhesively united layers of spirally wound strips, the innermost and the outermost strips being formed of di-electric kraft paper, at least one layer being formed of a spirally wound strip of a high temperature resistant polyamide.

2. The inductance of claim 1 in which the dielectric paper is impregnated with a flame-retardant phenolic resin.

3. The inductance of claim 1 in which the polyamide is nylon paper.

4. The inductance of claim 1 in which the layers are adhesively united.

5. The inductance of claim 1 in which the tubular member is impregnated with a flame-retardant phenolic resin and has a generally rectangular transverse cross section.

6. The inductance of claim 1 in which the tubular member is generally cylindrical and the innermost layer thereof is provided with radially inwardly extending projections, an externally threaded metal core being received within the tubular member, the threads of the core cooperating with the projections to hold the core within the tubular member and to permit the core to be moved axially with respect to the tubular member by rotating the core.

7. A coil form for use in providing an inductance comprising a plurality of adhesively united layers of spirally wound strips, the innermost and the outermost strips being formed of dielectric kraft paper, atleast one layer being formed of a spirally wound strip of a high temperature resistant polyamide.

8. The coil form claim 7 in which the dielectric kraft paper is impregnated with a flame-retardant resin.

9. The coil form of claim 7 in which the polyamide is nylon paper.

10. The coil form of claim 7 in which the dielectric kraft paper is impregnated with a flame-retardant phenolic resin and the coil form has a generally rectangular transverse cross section.

11. The coil form of claim 7 in which the coil form is generally cylindrical and is impregnated with a flameretardant phenolic resin, the innermost layer being provided with inwardly extending projections.

12. A method of making a coil form comprising the steps of spirally winding at least one strip of dielectric kraft paper to form a first tubular layer, spirally winding and adhesively uniting at least one strip of a high temperature resistant polyamide on the first tubular layer to form a second tubular layer, and spirally winding and adhesively uniting at least one strip of dielectric kraft paper on the second tubular layer to form a multilayered tube, impregnating the tube with a flame-retardant resin, and curing the impregnated tube.

13. The method of claim 12 including the step of winding an electrical conductor on the cured tube.

14. The method of claim 12 including the step of forming inwardly extending projections in the first layer after the tube is cured.

15. The method of claim 12 including the steps of indenting the tube after curing to form inwardly extending projections in the first layer and threading an externally threaded metal core into the tube.

16. A method of making a coil form comprising the steps of spirally winding at least one strip of di-electric kraft paper to form a first tubular layer having a generally circular cross section, spirally winding and adhesively uniting at least one strip of a high temperature resistant polyamide on the first tubular layer to form a second tubular layer having a generally circular cross section, and spirally winding and adhesively uniting at least one strip of dielectric kraft paper on the second tubular layer to form a multi-layered tube having a generally circular cross section, impregnating the tube with a flame-retardant resin, curing the impregnated tube, and forming the cured tube into a tube having a generally rectangular cross section.

17. The method of claim 16 in which the generally rectangular cross section of the tube is formed by advancing the tube axially within a die provided with a bore having a first generally circular portion and a second generally rectangular portion. 

2. The inductance of claim 1 in which the dielectric paper is impregnated with a flame-retardant phenolic resin.
 3. The inductance of claim 1 in which the polyamide is nylon paper.
 4. The inductance of claim 1 in which the layers are adhesively united.
 5. The inductance of claim 1 in which the tubular member is impregnated with a flame-retardant phenolic resin and has a generally rectangular transverse cross section.
 6. The inductance of claim 1 in which the tubular member is generally cylindrical and the innermost layer thereof is provided with radially inwardly extending projections, an externally threaded metal core being received within the tubular member, the threads of the core cooperating with the projections to hold the core within the tubular member and to permit the core to be moved axially with respect to the tubular member by rotating the core.
 7. A coil form for use in providing an inductance comprising a plurality of adhesively united layers of spirally wound strips, the innermost and the outermost strips being formed of dielectric kraft paper, at least one layer being formed of a spirally wound strip of a high temperature resistant polyamide.
 8. The coil form claim 7 in which the dielectric kraft paper is impregnated with a flame-retardant resin.
 9. The coil form of claim 7 in which the polyamide is nylon paper.
 10. The coil form of claim 7 in which the dielectric kraft paper is impregnated with a flame-retardant phenolic resin and the coil form has a generally rectangular transverse cross section.
 11. The coil form of claim 7 in which the coil form is generally cylindrical and is impregnated with a flame-retardant phenolic resin, the innermost layer being provided with inwardly extending projections.
 12. A method of making a coil form comprising the steps of spirally winding at least one strip of dielectric kraft paper to form a first tubular layer, spirally winding and adhesively uniting at least one strip of a high temperature resistant polyamide on the first tubular layer to form a second tubular layer, and spirally winding and adhesively uniting at least one strip of dielectric kraft paper on the second tubular layer to form a multi-layered tube, impregnating the tube with a flame-retardant resin, and curing the impregnated tube.
 13. The method of claim 12 including the step of winding an electrical conductor on the cured tube.
 14. The method of claim 12 including the step of forming inwardly extending projections in the first layer after the tube is cured.
 15. The method of claim 12 including the steps of indenting the tube after curing to form inwardly extending projections in the first layer and threading an externally threaded metal core into the tube.
 16. A method of making a coil form comprising the steps of spirally winding at least one strip of di-electric kraft paper to form a first tubular Layer having a generally circular cross section, spirally winding and adhesively uniting at least one strip of a high temperature resistant polyamide on the first tubular layer to form a second tubular layer having a generally circular cross section, and spirally winding and adhesively uniting at least one strip of dielectric kraft paper on the second tubular layer to form a multi-layered tube having a generally circular cross section, impregnating the tube with a flame-retardant resin, curing the impregnated tube, and forming the cured tube into a tube having a generally rectangular cross section.
 17. The method of claim 16 in which the generally rectangular cross section of the tube is formed by advancing the tube axially within a die provided with a bore having a first generally circular portion and a second generally rectangular portion. 